1. Technical Field
The present disclosure relates to a magnetoresistive memory device and a writing method thereof. More particularly, the present disclosure relates to a ring-shaped magnetoresistive memory device and a writing method thereof.
2. Description of Related Art
A magnetoresistive random access memory (MRAM) is non-volatile memory. The MRAM is capable reading and writing at a high speed as that of static random access memory (SRAM), and has a high record density as a dynamic RAM (DRAM). In this regard, the MRAM has become one of the most important developments for future memories.
The MRAM can have a plurality of memory cells. One of the simplest structures of memory cells may have three layers, which in order are a magnetic layer, a separation layer and another magnetic layer. One of the two magnetic layers is employed as a data layer for storing data, which is also called a free layer. The other magnetic layer is employed as a fixed layer, which is also called a pinned layer, and a magnetization direction thereof is fixed (pinned). The separation layer disposed between the two magnetic layers can be made of non-magnetic metallic material or non-conductive (electrically insulating) material. The non-magnetic metallic material may be made of a giant magnetoresistance (GMR)-based component; the non-conductive material may be made of a tunneling magnetoresistance (TMR)-based component. When a magnetization direction of the data layer is identical to the magnetization direction of the fixed layer, the memory cell of the MRAM is in a low resistance state. When the magnetization direction of the data layer is contrary to the magnetization direction of the fixed layer, the memory cell of the MRAM is in a high resistance state. Therefore, digital signals of “0” or “1” are recorded in response to the two resistance states of the memory cell of the MRAM.
Furthermore, the memory cell has a dimension about several tens to hundreds of nanometers of an anisotropic oval shape, so as to keep the data stability thereof. However, a record density of the oval-shaped memory cell is limited due to the magnetostatic interaction between the adjacent oval-shaped memory cells. To address the problem, a ring-shaped memory cell is provided. When a plurality of magnetic moments of the ring-shaped memory cell is circularly arranged (in a circulation state), there are no magnetic charges generated at the boundary of the ring-shaped memory cell, and the magnetostatic interaction between the adjacent oval-shaped memory cells can be avoided. As such, the limitation of the record density of the ring-shaped memory cell can be avoided accordingly. Therefore, the ring-shaped memory cell becomes the first choice for enhancing the record density.
Although the record density of the ring-shaped memory cell can be enhanced, how to write data for the ring-shaped memory cell is still a difficult problem. In case of the circularly arranged magnetic moments of the ring-shaped memory cell, the orientations of the magnetic moments thereof along a counterclockwise direction or along a clockwise direction are used to represent “0” or “1” respectively. Therefore, it is critical to control the orientations of the magnetic moments with a low energy-consuming method for practical application.
At present, the writing methods of the MRAM can be sorted into three categories: magnetic writing method, current writing method, and a thermally assisted writing method (TA-MRAM). The magnetic writing method is base on a traditional X-Y selection by selecting an address line to generate a magnetic field for changing a magnetization direction of a memory cell. The required magnetic field is increased with the reduced dimension of the memory cell, which consumes more energy and may possible burn out the memory cell. The current writing method changing the magnetization direction of a memory cell by spin transfer torque generated from a spin-polarized current. However, the spin-polarized current required for the current writing method is still high, and it is required a highly advanced technique to make a magnetic tunnel junction of the kind of memory cell. Therefore, the current writing method has drawbacks of high manufacturing difficulty and high cost. Further, the thermally assisted writing method heats a magnetic tunnel junction by using a direct current so as to reduce the coercivity of the free layer. Comparing with the former two writing methods, the thermally assisted writing method may generate a magnetic field with less writing current.
A writing method for the MRAM is disclosed in U.S. Pat. No. 6,545,906 B1, in which a magnetization direction of a free layer of an oval-shaped memory cell is changed by a “toggle write mode”. In this disclosure, the free layer of the MRAM uses a synthetic antiferromagnet (SAF) structure, i.e., a thin metal layer of Ru that is disposed between two magnetic layers, and an angle between a long axis of the oval-shaped memory cell and an address line for writing is 45 degrees. Accordingly, a specific manufacturing process is required for satisfying the above limitations of structure of the memory cell and the writing method, which limits the scope of the applications for the technique.
Moreover, a writing method of a ring-shaped memory cell is provided by Jian-Gang Zhu, Youfeng Zheng and Gary A. Prinz in Carnegie Mellon University (J. Appl. Phys. 87, 6668 (2000)). The orientations of the magnetic moments are controlled by an in-plane field. First, a hole of the ring-shaped memory cell is deviate from a central position. Then a small cut is formed at the ring portion of the ring-shaped memory cell. Also, the manufacture of the ring-shaped memory cell highly depends on delicate and complicated process steps, and some of them may even need the application of an e-beam lithography. As a result, the writing method has drawbacks of difficulty manufacturing process, high cost and an unsatisfied yield ratio.
Given the above, the manufacturing techniques and writing methods of the conventional memory device both met the problems of complicated and difficult manufacturing process, so that the manufacturing yield is hard to be enhanced, and the cost is kept high.